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    <title>Document</title>
    <style>
        /* 既有A标签又有B类  A.B */
        /* 之间不要空格！！！ */
        h1.c1{color: blue;}
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    <h1>asudfadsuf</h1>
    <h1 class="c1">asudfadsuf</h1>
</body>
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